Don't Hold The Connection Of Session

hello, i have an old ver of Cs-cart 4.15 an work ok until 4-5 days ago.

Some clients have report that even the password is correct, can't login.

Same pass from other pc may login or not. In same Pc, IE can login but Chrome can't and

in other pc Chome Can login (same ver)

That from chome and from mobile IOS / Android.

But if client try to connect from private window, then can login correctly and keep the session.

Any ideas?

Thank you for your time.